Cardiopulmonary bypass (CPB) leads to profound alterations in the coagulation and immune systems, resulting in clinical complications including bleeding, thrombosis, and reperfusion injury. As demonstrated under the prior aegis of this grant, the cellular components of the two systems do not operate independently but rather directly interact to affect mutual function, as well as interacting with soluble mediators. The underlying hypothesis is that CPB leads to dynamic alterations in plateIet- Ieukocyte-endothelial cell adhesive nd functional interactions contributing to pathophysiologic events. Published articles have shown that (1) platelets are activated in vivo by CPB with resultant selective changes in adhesive ligands on the platelet surface membrane; (2) these changes are more profound in specific patient subpopulations, e.g., pediatric cyanotic congenital heart disease patients; (3) this activation leads to formation of in vivo circulating platelet leukocyte (plt-wbc) conjugates and, in temporal parallel, upregulation of the leukocyte adhesive receptor CD1 1 b on wbc; (4) pharmacologic agents such as acadesine modulate these changes; (5) adhesion of activated platelets to wbc is mediated by platelet P- seIectin while resting platelet binding is mediated by a different receptor; (6) an in vitro whole blood model shows a dynamic interplay of plt-plt homotypic adhesion versus plt-wbc heterotypic adhesion as well as dynamic alterations in plt-wbc adhesion over time that mimic he in vivo physiology; (7) aspirin does not block platelet o-granule release and consequent P-selectin expression and function. Ongoing studies show that (1) platelet adhesion results in a wbc signal transduction event ([Ca+2] flux); (2) activated platelet adhesion to monocytes results in functional tissue actor upregulation; (3) fibrinolytic protease inhibitors alter plt- wbc activation in an ex vivo "closed loop" CPB model; (4) other clinical conditions - pre-eclampsia and cocaine abuse - mimic part of the pathophysiology of CPB, as does platelet transfusion. Specific questions for the next proposed grant period are: (1) what are the quantitative and functional changes in (a) platelet and (b) leukocyte adhesion receptors and in plt-wbc conjugate formation during clinical CPB?; (2) what are the in vitro and in vivo functional consequences of wbc adhesion, including (a) early signal transduction events such as [Ca +2] flux and (b) late effector events such as oxidative burst, CD1 1 /CD11/CD18 upregulation, and tissue factor expression; what are the molecules on the plt and wbc surface membranes that mediate these events?; (3) what are the mechanisms underlying these changes in CPB?; what physiological and pharmacological mediators alter these parameters?; what is the role of the endothelium and of shear stress in these interactions? The long term goals are to (1) describe he pathobiology of CPB, (2) understand the normal molecular mechanisms underlying this biology, and (3) determine potential therapeutic interventions to abrogate clinical complications of the procedure.